Phosphorylation of myosin from proliferative myoblasts is a pre-requisite for actin-activation of this myosin. The myosin light chain kinase responsible for the phosphorylation of the 20,000 dalton light chain in these cells is not dependent on Ca 2 ion for its activity. Following cell fusion, the kinase disappears. As the cells develop sarcomeres, a Ca 2 ion dependent kinase is synthesized. The myosin isozyme in these cells is similar to the adult rat thigh muscle myosin.